Field of the Invention
The present invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus including, in an image sensor, an A/D converter for converting an analog signal obtained by photoelectric conversion into a digital signal.
Description of the Related Art
Recent image pickup apparatuses such as a digital camera and digital video camera require a technique of reading out image signals from an image sensor at high speed to increase the number of pixels and improve the continuous shooting speed. When performing, for each pixel, analog/digital conversion (A/D conversion) for a signal read out at high speed, the conversion time for each pixel data is very short. It is, therefore, necessary to perform conversion with high accuracy within a short time.
To solve this problem, research and development have been carried out for a column A/D conversion image sensor in which A/D convertors are arranged for each column of pixels of a CMOS image sensor which can be manufactured by the same process as that of a CMOS integrated circuit to simultaneously A/D-convert pixel signals on one row. In the column A/D conversion image sensor, the conversion rate of the A/D convertor can be lowered from a readout rate for each pixel to that for each row, thus readily speeding up the readout rate of one frame of the image sensor.
Some column A/D conversion image sensors adopt a ramp A/D conversion method of causing a comparator to compare a ramp signal with a pixel signal, and measuring the time until the output of the comparator is inverted, as described in Japanese Patent Laid-Open No. 05-048460. This ramp A/D conversion method, however, needs to count clocks by the number of conversion bits. Therefore, a large number of bits significantly prolongs the A/D conversion time. For example, the A/D conversion time for (N+2) bits is four times that for N bits.
As a method of solving this problem, a method of changing the relative relationship between a signal amplitude and a ramp signal amplitude in accordance with a signal amplitude and performing A/D conversion is plausible, as disclosed in Japanese Patent Laid-Open No. 2010-45789.
In Japanese Patent Laid-Open No. 2010-45789, in N-bit A/D conversion, the value of an analog signal is compared with a threshold obtained by dividing the full-scale amplitude by 2K (K is an integer smaller than N). The signal is A/D-converted into (N−K) bits. If the value of the signal is larger than the threshold, the result of the A/D conversion is set at (N−K) bits on the MSB side of N-bit digital data. Alternatively, if the value of the signal is equal to or smaller than the threshold, the result is set at (N−K) bits on the LSB side of N-bit digital data. This increases the number of bits of A/D converter while suppressing an increase in A/D conversion time. Furthermore, Japanese Patent Laid-Open No. 2010-45789 discloses the following arrangement. That is, two ramp signals having different slopes are generated, and if the signal value is larger than the threshold, a ramp signal having a larger slope is used to perform conversion; otherwise, a ramp signal having a smaller slope is used to perform conversion.
As described in Japanese Patent Laid-Open No. 2010-45789, however, when the relative magnitudes of the signal and the ramp signal are changed based on the result of comparing the signal value with the threshold, the characteristics (input/output characteristics) of final output data with respect to the analog input signal may shift at the threshold. FIG. 22 schematically shows an example of the input/output characteristics. Let Vm be a threshold for switching an A/D conversion method by comparison with the value of a signal. A dotted line indicates ideal input/output characteristics, and thick solid lines indicate actual input/output characteristics. It is desirable that the actual input/output characteristics coincide with the ideal input/output characteristics. When switching between the ramp signals in accordance with the magnitude relationship between the threshold and the signal value as described in Japanese Patent Laid-Open No. 2010-45789, the slopes of the two ramp signals and the occurrence of an offset may cause a shift in the input/output characteristics at the threshold Vm as a boundary, as shown in FIG. 22.